1. Field of the Invention
The present invention relates to a band reduction or compression system, and more particularly to a coding system for the transmission and reception of an image signal obtained by scanning a two-dimensional image for facsimile transmission and the like.
2. Description of the Prior Art
There has been proposed a system for collectively coding a plurality of images by utilizing two-dimensional correlation in both the horizontal and the vertical directions of scanning, since a high compression rate to band reduction cannot be used with one-dimensional correlation of an image. As an example thereof, there is employed a system for performing band compression by detecting a black and white mode between a predetermined number of adjacent picture elements, giving a code to each mode, measuring run length of each mode, and coding the run length.
When employing such system, however, a plurality of scanning lines are collectively coded, so that if a signal error is generated in a transmission line, an image reproduced in said scanning line after generation of the error becomes an ugly image of inferior reproducibility apart from the original image, that is, the reproduced image by the scanning line where an error is generated makes all the picture elements in the scanning line white or becomes the same reproduced image as that immediately before the scanning line. When a plurality of scanning lines are reproduced, a considerably deteriorated image is obtained, and further since a plurality of scanning lines are collectively transmitted, there is required such steps that a mode of the corresponding position is detected, transition to the next mode is detected, a transition code is given thereto, and a run length of the mode is coded. Then, each mode often has a run length of 1, so that if a transition code is given to each mode and a code expressing the run length is given, redundancy is inevitably increased, and as a result, the apparatus becomes very complicated and expensive.